plans: tick calculator.hs + 5/5 classic programs target

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-on-sx.md

@@ -86,9 +86,9 @@ Key mappings:
   - [x] `sieve.hs` — lazy sieve of Eratosthenes
   - [x] `quicksort.hs` — naive QS
   - [x] `nqueens.hs`
-  - [ ] `calculator.hs` — parser combinator style expression evaluator
+  - [x] `calculator.hs` — parser combinator style expression evaluator
 - [ ] `lib/haskell/conformance.sh` + runner; `scoreboard.json` + `scoreboard.md`
-- [ ] Target: 5/5 classic programs passing
+- [x] Target: 5/5 classic programs passing
 
 ### Phase 4 — Hindley-Milner inference
 - [ ] Algorithm W: unification + type schemes + generalisation + instantiation
@@ -114,6 +114,15 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-25** — Classic program `calculator.hs`: recursive descent
+  expression evaluator using ADTs for tokens and results.
+  `data Token = TNum Int | TOp String` + `data Result = R Int [Token]`;
+  parser threads token lists through `R` constructors enabling nested
+  constructor pattern matching (`R v (TOp "+":rest)`). Handles two-level
+  operator precedence (* / tighter than + −) and left-associativity.
+  5 tests: addition, precedence, left-assoc subtraction, left-assoc
+  div+mul, single number. All 5 classic programs complete. 402/402 green.
+
 - **2026-04-25** — Classic program `nqueens.hs`: backtracking n-queens via list
   comprehension and multi-clause `where`. Three fixes needed: (1) `hk-eval-let`
   now delegates to `hk-bind-decls!` so multi-clause `where`/`let` bindings